Lung epithelium is prone to frequent and recurrent injuries from inhaled pathogens and pollutants. Understanding the regenerative capacity and the role of resident alveolar stem and progenitor cells is therefore of considerable practical and therapeutic interest. Repair of damaged alveoli is crucial for restoration of normal lung function but the cellular and molecular mechanisms regulating repair are still not well understood. We have identified a novel lung stem/progenitor population in the distal alveoli that is marked by Sox9 and that these cells are anatomically and molecularly distinct from the known alveolar progenitor cells. We also find that Sox9 expressing cells contribute to the distal alveoli followin influenza injury and that the expression of SOX9 protein us increased following injury. Based on this preliminary data, we hypothesize that these novel Sox9+ cells are multipotent plastic progenitor cells of the alveoli that are essential for airway and alveolar injury repair. We also hypothesize that the transcription factor SOX9 is required to regulate injury repair by co-operating with key lung-specific transcription factor to regulate adult alveolar homeostasis and regeneration. To test these hypotheses, we propose the following specific aims. In the K99 phase of the proposal, we will 1) determine the role of a novel population of Sox9-expressing cells in alveolar homeostasis and in alveolar injury repair; 2) determine the effect of gain and loss of function of SOX9 on adult alveolar homeostasis and regeneration. In the ROO phase of this application, we will define the molecular mechanisms through which SOX9 regulates adult alveolar maintenance and regeneration. We will simultaneously use lineage tracing and cell ablation murine models in combination with influenza injury to study the contribution of Sox9+ cells to regenerated lung. We will also study the effects of Sox9 gain and loss of function using conditional ectopic expression and loss of function mice, respectively. Finally, we will identify Sox9 interacting proteins and Sox9 bound genomic loci using co-immunoprecipitation and Chromatin IP experiments, respectively, followed by functional analysis of target proteins and genes. We will also address the question whether activation of these novel cells and molecular mechanisms accelerates alveolar repair, making these cells therapeutically relevant. I am a postdoctoral fellow at the Center for Regenerative Medicine, Massachusetts General Hospital. From my masters, Ph.D. and current postdoctoral training I have gained substantial research experience in stem cell and molecular biology. I have gained substantial expertise in developing new mouse models of injury, identifying novel genes and novel genetic and epigenetic transcription regulatory elements. In recent years, I have developed novel tools for modulating gene expression specific to lung tissues and identified a remarkable cellular plasticity phenomenon in regenerating airway in which differentiated cells can become stem cells after injury. In the current proposal I seek support to study a novel putative stem cell population and the associated transcriptional and signaling pathways in the alveoli. I intend to gain both scientific and career development skills from my mentor Dr. Jayaraj Rajagopal and co-mentor Dr. David Scadden. Dr. Rajagopal has all the necessary stem cell and mouse genetics expertise as well as the experience with Sox proteins and Wnt signaling to guide my early research plan and I have already developed some of the expertise to handle the biochemical studies proposed in the R00 phase of this grant. I chose Dr. Rajagopal's lab since he is considered one of the rising stars in lung stem cell biology. But in the interests of having a breadth of mentorship, I wll take advantage of the expertise from of Dr. David Scadden, a renowned stem cell biologist and director of the Harvard Stem Cell Institute that himself has fostered the career of many independent PIs, Dr. Konrad Hochedlinger, an expert in transcriptional regulation and Sox-proteins, Dr. Wellington Cardoso, an expert in airway and lung specific signaling pathways, and Dr. Darrel Kotton, an expert in alveolar biology. The rich research environment and facilities at the Massachusetts General Hospital, Harvard Stem Cell Institute, Harvard University, and Harvard Medical School and the expert guidance from my mentors and advisory members will help me to develop new skills to establish as an independent investigator. I will devote my 100% of my time to focused research program and to training activities including attendance and presentation at both scientific and career developmental activities.